docs: def download(self, attr):
          """
          Download an attribute attachment
          (if type is malware-sample or attachment only)
          :param attr: attribute (should be MispAttribute instance)
          :returns: value of the attachment
          """
          if attr.type not in ['malware-sample', 'attachment']:
              raise ValueError('Only malware-sample and attachment can be downloaded')
  
          return self.GET('/attributes/downloadAttachment/download/%i' % attr.id)